A crankshaft is a principal component of a reciprocating engine, which power is extracted by converting reciprocating motion of pistons to rotary motion. Generally, there are two types of crankshafts: those that are manufactured by forging and those that are manufactured by casting. For 4-cylinder engines for automobiles such as passenger cars, freight cars, and specialized work vehicles, it is necessary that their crankshafts have high strength and stiffness, and therefore forged crankshaft, which are more capable of meeting the need, are widely used. For 4-cylinder engines of motorcycles, agricultural machines, marine vessels, and the like, forged crankshafts are also used.
In general, forged crankshafts for 4-cylinder engines are manufactured by using, as a starting material, a billet, and subjecting the billet to the steps of preforming, die forging, trimming and coining in order. The billet has a circular or square cross section and has a constant cross-sectional area along the overall length. The preforming step includes roll forming and bending, and the die forging step includes block forging and finish forging.
FIG. 1 is a schematic diagram illustrating a typical conventional process for manufacturing a forged crankshaft for 4-cylinder engines. A crankshaft 1 illustrated in FIG. 1 is intended to be mounted in a 4-cylinder engine. It is a 4-cylinder 8-counterweight crankshaft that includes: five journals J1 to J5; four crank pins P1 to P4; a front part Fr, a flange Fl, and eight crank arms (hereinafter referred to as “crank arms”) A1 to A8 that connect the journals J1 to J5 and the crank pins P1 to P4 to each other. This crankshaft 1 is a 4-cylinder 4-counterweight crankshaft. Among the eight arms A1 to A8, the first and eighth arms A1, A8 at opposite ends, and the fourth and fifth crank arms A4, A5 connecting with a central third journal have balance weights. The second, third, sixth, and seventh arms A2, A3, A6, and A7 have no balance weights, therefore having an oval shape. Hereinafter, when the journals J1 to J5, the crank pins P1 to P4, and the crank arms A1 to A8 are each collectively referred to, a reference character “J” is used for the journals, a reference character “P” for the crank pins, and a reference character “A” for the crank arms. An arm having a balance weight is also referred to as a weighted arm when distinguished from an arm having no balance weight. On the other hand, an arm having no balance weight is also referred to as a non-weighted arm or an oval arm.
According to the manufacturing method shown in FIG. 1, the forged crankshaft 1 is manufactured in the following manner. Firstly, a billet 2 shown in FIG. 1(a), which has been previously cut to a predetermined length, is heated by a heating furnace and then is subjected to roll forming. In the roll forming step, the billet 2 is rolled and reduced in cross section by grooved rolls, for example, to distribute its volume in the longitudinal direction, whereby a rolled blank 103, which is an intermediate material, is formed (see FIG. 1(b)). In the bending step, the rolled blank 103 obtained by roll forming is partially pressed in a press in a direction perpendicular to the longitudinal direction to distribute its volume, whereby a bent blank 104, which is a secondary intermediate material, is formed (see FIG. 1(c)).
Then, in the block forging step, the bent blank 104 obtained by bending is press forged with a pair of upper and lower dies, whereby a forged blank 105 having a general shape of a crankshaft (forged final product) is formed (see FIG. 1(d)). Then, in the finish forging step, the block forged blank 105 obtained by block forging is further processed by press forging the block forged blank 105 with a pair of upper and lower dies, whereby a forged blank 106 having a shape in agreement with the shape of the crankshaft is formed (see FIG. 1(e)). In the block forging and the finish forging, excess material flows out as a flash from between the parting surfaces of the dies that oppose each other. Thus, the block forged blank 105 and the finish forged blank 106 have large flashes 105a, 106a, respectively, around the formed shape of the crankshaft.
In the trimming step, the finish forged blank 106 with the flash 106a, obtained by finish forging, is held by dies from above and below and the flash 106a is trimmed by a cutting die. In this manner, the forged crankshaft 1 is obtained as shown in FIG. 1(f). In the coining step, principal parts of the forged crankshaft 1, from which the flash has been removed, e.g., shaft parts such as the journals J, the crank pins P, the front part Fr, and the flange Fl, and in some cases the crank arms A, are slightly pressed with dies from above and below and formed into a desired size and shape. In this manner, the forged crankshaft 1 is manufactured.
The manufacturing process shown in FIG. 1 is applicable not only to a 4-cylinder 4-counterweight crankshaft as exemplified, but also to other 4-cylinder 4-counterweight crankshaft. In another type of 4-cylinder 4-counterweight crankshaft, among the eight arms A, in place of the leading first arm A1 connecting with a first pin P1 at the fore end, the second arm A2 connecting with the same first pin P1 has a balance weight. In this crankshaft, in place of the trailing eighth arm A8 connecting with the fourth pin P4 at the rear end, the seventh arm A7 connecting with the same fourth pin P4 has a balance weight.
With such a manufacturing method, it is inevitable that material utilization decreases because large amounts of unnecessary flash, which is not a part of the end product, are generated. Thus, in the manufacturing of a forged crankshaft, it has so far been an important object to inhibit the generation of flash to the extent possible and achieve improvement of material utilization. Examples of conventional techniques that address this object are as follows.
For example, Japanese Patent Application Publication No. 2008-155275 (Patent Literature 1) and Japanese Patent Application Publication No. 2011-161496 (Patent Literature 2) disclosure techniques for manufacturing a crankshaft, by which journals and crank pins are shaped and crank arms are roughly shaped. In the technique Patent Literature 1, a stepped round bar having reduced diameter regions at portions to be formed into journals and crank pins of a crankshaft is a round bar used as a blank. A pair of the portions to be formed into journals, between which a portion to be formed into a crank pin is disposed, are held with dies. In this state, the opposing dies are axially moved toward each other to compressively deform the round bar blank. Concurrently with imparting this deformation, punches are pressed against the portion to be formed into a crank pin in a direction perpendicular to the axial direction, whereby the portion to be formed into a crank pin is placed into an eccentric position. The above operation is repeated in succession for all crank throws.
In a technique of Patent Literature 2, a simple round bar is used as a blank. One end of the two ends of the round bar is held with a stationary die and the other end thereof is held with a movable die, and a portion to be formed into a journal is held with journal dies and portions to be formed into crank pins with crank pin dies. In this state, the movable die, the journal dies and the crank pin dies are axially moved toward the stationary die to compressively deform the round bar blank. Concurrently with imparting this deformation, the crank pin dies are moved in an eccentric direction perpendicular to the axial direction to place the portion to be formed into the crank pin into an eccentric position.
With both the techniques of Patent Literatures 1 and 2, no flash will be generated, and therefore a significant improvement in material utilization can be expected.